1. Field of the Invention
The present invention generally relates to the field of telecommunications, and more particularly to wireless, mobile communications networks, such as mobile telephony networks. Specifically, the invention concerns the distribution of relevant amounts of data, such as multimedia information contents, to mobile user terminals (e.g., mobile phones) through a wireless, mobile communications network.
2. Description of the Related Art
Mobile telephony networks were initially conceived for enabling voice communications, similarly to the wired, Public Switched Telephone Networks (PSTNs), but between mobile users.
Mobile telephony networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese counterparts).
The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS) and Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available in the last few years.
However, these second-generation cellular networks, albeit satisfactory for voice communication, offer very poor data exchange capabilities.
Similarly to the PSTNs, second-generation cellular networks are in fact switched-circuit networks; this greatly limits the bandwidth that can be allocated for a given user. On the contrary, data communications networks such as computer networks and, among them, the Internet, adopt packet switching schemes, which allow much higher data transfer rates.
Some solutions have been proposed to overcome the limitations of conventional, switched-circuit cellular networks such as the GSM networks, so as to enable users of mobile terminals efficiently exploiting services offered through the Internet.
One of the solutions that have acquired a significant popularity is the General Packet Radio Service (shortly, GPRS). The GPRS is a digital mobile phone technology compatible with GSM networks (actually, built on the existing GSM network architecture) that enables data transfer at a speed higher than that allowed by pure GSM.
Essentially, the GPRS can be viewed as a GSM add-up that supports and enables packet-based data communication.
Although third-generation wireless communications systems such as those complying with the Universal Mobile Telecommunication System (UMTS) are more promising in terms of data transfer rates, the GPRS is a ready-at-hand solution for enhancing the data exchange capabilities of already existing GSM networks, and is therefore gaining an increasing popularity.
In GPRS communications networks the information content is usually transferred in a point-to-point modality (unicasting), upon activation of a session between a GPRS mobile phone (or mobile station) and a service provider connected to a packet data network, e.g. a server connected to the Internet; the activation of such a session involves the setting up of logic connections between the server and the GPRS mobile phone.
In such a point-to-point communication mode, the radio resources to be allocated for the exchange of data between the ground GPRS network and the GPRS mobile stations depend on the number of different mobile stations simultaneously exploiting the GPRS services, even if the same GPRS service is being exploited by two or more mobile station users at the same time. Clearly, this limits the possibility of simultaneously accessing available GPRS services by several users, unless the radio resources are overdimensioned.
Thus, it would be desirable to have the possibility of delivering information contents related to a same GPRS service exploitable by two or more users at a time through a point-to-multipoint transmission mode, saving the amount of allocated resources.
The problem of broadcasting relatively massive information contents, such as multimedia (audio and/or video) contents, to several mobile terminal users has already been faced in the art.
In particular, the 3GPP (3rd Generation Partnership Project) Technical Specification No. TS 23.246 (“Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description”), v6.1.0, December 2003, whose teachings are herein incorporated by reference, describes the architectural solution and functionalities for MBMS.
The above-referred Technical Specification deals with the “core network” aspects, and does not cover the physical, radio communications aspects involved in the implementation of a MBMS.
Multicast transmission in GPRS networks is also dealt with in the International application No. WO 03/019840 A2. The method proposed in that document provides for routing through a cellular network only a single copy of multicast data to a plurality of mobile stations in a cell; a virtual subscriber mobile station is emulated, that subscribes to a multicast service and receives the multicast data for the cell. The real mobile stations that should receive the multicast data are instructed to listen to, and consequently tune to a channel assigned to the virtual subscriber.
The Applicant observes that the implementation of the proposed method is not free of costs, because it involves modifying in a significant way the network apparatuses governing the radio communications. In particular, elements are needed to emulate the virtual mobile station.
Furthermore, the Applicant observes that the proposed method shows an additional and more severe drawback: the different mobile stations in a cell that are exploiting a given multicast service are indistinguishable for the network apparatuses, which are thus impossibilitated to address information to a specific mobile station.
In the Applicant opinion, this is a severe limitation, for example because it makes impossible to implement efficient policies of acknowledge/not-acknowledge of the distributed content.
According to a submission recently made at the GERAN2 #18 Meeting, held in Phoenix, Ariz., U.S.A. from Mar. 22 to Mar. 26, 2004 (the text of which can be downloaded from the Internet site ftp://ftp.3gpp.org/TSG_GERAN/WG2_protocol_aspects/GERAN2—18bis_Phoenix/Docs/G2-040286, entitled “Common Feedback Channel for MBMS delivery”, there is proposed to define a Common Feedback CHannel (CFCH) intended to be used as a feedback channel, where negative acknowledgments (nack) are sent as access bursts at precise times. More specifically, according to the proponents, feedback messages are sent by all interested Mobile Stations (MSs) as access bursts on the CFCH at a precise time: if MS does not decode the RLC block transmitted at time t, it will send an access burst at time t+Δt; if a MS successfully decodes the RLC block transmitted at time t, nothing is transmitted on the feedback channel at time t+Δt.
The consequence is that, if an access burst is detected at time t+Δt (the network could infer that one or more MSs have sent a nack from the increased received power on the feedback channel), the network realizes that the block transmitted at time t has not been received (at least) by one MS.
The proponents observe that if several MS's send access bursts at the same time and they collide, there might be no problem since they all carry the same info (i.e. loss of block sent at time t). The information is not the content of the access burst, but the presence of the access burst itself.
The Applicant however is of the opinion that this implementation of the feedback from the MSs is not suitable to implement efficient acknowledge/not-acknowledge policies. For example, this implementation does not allow to recognize if a series of nack responses may be ignored because it is coming from a mobile station located in a geographical area in which the signal reception is very poor. As another example, this implementation cannot allow a better tuning of some parameters of the mobile stations in order to increase the quality of the reception of the data contents, such as for example the Timing Advance of the mobile stations. It is also noticed that the proponents of the cited solution clearly state that their goal is not to realize a fully acknowledged protocol.
The Applicant has faced the problem of implementing a MBMS service in which information may be specifically addressed to different mobile stations (for example an ack/nack request), and in which information from the mobile stations can be recognized and distinguished (for example, in order to get an ack/nack feedback), by suitable network equipment.